1. Field of the Invention
The present invention generally relates to an image processing apparatus, and more particularly to an image distortion correction apparatus, an image distortion correction method therefor, recording media, an image scanner and an image construction apparatus, in which a distorted part of a scanned image is corrected.
2. Description of the Related Art
When a book type original, for example, a book or a booklet, is copied or scanned, it is hard to keep a binding part of the book on a focus plane of a copier or a scanner. The binding part of the book type original is separated from the focus plane of a copier or a scanner. This causes a copied or scanned image to be distorted and sharpness of characters on the copied or scanned image is reduced. The distorted part of the copied or scanned image is hard to read and, furthermore, a character recognition rate of an OCR (optical character reader) becomes very low. Above all, the thicker the book is, the harder it is to read the copied or scanned image. On the other hand, if the book is pressed to the focus plane, the book may be damaged.
To address this problem, therefore, a method to correct a distorted image has been proposed, in which method a three dimensional shape of an object is estimated based on density information of the image. An example of that method is described in a publication of T. Wada, H. Uchida and T. Matuyama, “Shape from Shading with Interreflection under a Proximal Light Source: Distortion-Free Copying of an Unfolded Book”, International Journal Computer Vision 24(2), 125–135(1997) and the representative method of shape from shading is further described in a publication of Hiroyuki Ukita and Katsunobu Konishi, “Three dimensional shape recovery for a booklet surface using intrinsic space method”, IECE technical report, PRMU99–91, pp. 23–30, October, 1999.
Further, a method is proposed in the laid-open Japanese patent application number 11-41455, in which method a three dimensional shape of an object is estimated based on an outline of a scanned image.
However, it is hard to put the method called the shape from shading into practical use because the method requires significant computational steps, and therefore requires a computational time of long duration.
On the other hand, a method described in the laid-open Japanese patent application number 11-41455 only requires a few computational steps to correct the distorted image, however, it is difficult to sufficiently correct the distorted image using this method when the whole outline of the booklet is not fully captured.